Add a default clear mode

[mitsuhiko]

ProgressBar::finish should clear with a style default setting. This would also help Drop automatically clear progress bars if wanted.

eg:

let style = ProgressStyle::new().clear_on_finish(true);

Then the progress bar will automatically disappear if someone calls finish() on it.

[djc]

We now have 6 different finish modes that can be used, so I believe this is fixed.